1. Field of the Invention
The present invention generally relates to the estimation of the flow noise on a cylindrical body in a turbulent flow and, more particularly, to a method for determining inner and outer boundary layer length scales from a succession of drag measurements of a long thin cylindrical body in any fluid as a precursor to estimating flow noise.
2. Description of the Prior Art
There is a significant practical need to know the drag and flow noise of towed long thin cylindrical bodies. The need arises in a variety of contexts including torpedoes and towed sonar arrays.
Towed sonar arrays are sonar systems that are designed to be towed by a submarine or a surface vessel in order to detect other submarines. The arrays are typically long, hose-like structures measuring up to a thousand feet or longer that contain specially designed acoustic sensors, called hydrophones, which receive acoustic waves. The arrays include electronics that convert the acoustical waves from analog to digital form and transmit that data to electronic processors on board the towing vessel.
The processor must distinguish radiated sound from other submarines from ambient and self noise, which includes the flow noise of the towed array. Thus, it is important to accurately estimate flow noise in advance, for design purposes. Moreover, towed arrays must be designed to withstand the extreme environmental stresses of operation in the ocean depths, and so it is necessary to accurately estimate drag, and estimate the local wall shear stress as well. Accomplishing this requires an understanding of the turbulent boundary layers which exist on the arrays.
The inner region of the boundary layer is dominated by viscous effects, and the outer region is dominated by inertial effects. Two dimensional flat plate turbulent boundary layers have been explored thoroughly for several decades, and it is generally accepted that the (inner) viscous length scale and the (outer) boundary layer momentum thickness scale adequately characterize the flow.
Most practical engineering flows, however, are characterized as high-Reynolds number flows. Since the viscous length scale decreases rapidly with increasing Reynolds number, and the outer length scales are only a weak function of Reynolds number, the inner and outer scales become increasingly disparate with increasing Reynolds number. Thus, more complex turbulent flows are often not well described by the Reynolds number alone, and must be described using inner and outer boundary layer length scales.
In the context of a towed array, the hydrodynamic flow is a high Reynolds number turbulent boundary layer, which may be equilibrium or nonequilibrium depending on the ship motion. Consequently, it is necessary to know the inner and outer boundary layer length scales, which characterize the flow field, for estimation of flow noise on long thin cylinders, and in particular, current and next generation towed sonar arrays.
Currently there are no viable approaches for determining the inner and outer boundary layer length scales in tow tank testing or full scale sea trials. Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV) have been used extensively for measurements of turbulence in laboratories. However, oceanic field applications are impractical. It would be greatly advantageous to provide a method for determining inner and outer boundary layer length scales and, more particularly, from a succession of drag measurements of a long thin cylindrical body, in order to estimate flow noise and for improved computational modeling of the dynamics of towed arrays in water or other towed bodies in air.